Recently, services for distributing contents in a digital form such as movies and music over the Internet or the like have been offered. Such contents are encrypted using a digital rights management (DRM) technology and then distributed. The DRM technology prohibits the contents from being illegally copied or distributed.
Unfortunately, incidents have occurred in which a viewer has illegally copied by photographing a picture content displayed on a computer display or a television monitor using filming equipment such as a camcorder, and has illegally distributed the copy of the picture content. Picture contents displayed on a display are not encrypted, and therefore it is difficult to prevent the picture contents copied by photographing from being distributed.
To address this, technologies of embedding in advance information, such as an identification number of a viewer, as a digital watermark in the content are being developed. Even if, for example, the picture content with a digital watermark embedded therein that is displayed on a display is photographed using a camcorder to be illegally copied, the digital watermark still remains in the copy of the picture content. After such a picture content illegally copied by capturing a picture in analog form has been distributed, it is possible to obtain the identification number of a viewer who has illegally used the picture content by detecting a digital watermark from the copy of the picture content. Accordingly, even in cases where the picture content is illegally copied by capturing a picture in analog form and is uploaded to a posting site or the like, the administrator of the picture content can specify where the picture content has come from, from information embedded in the uploaded picture content.
Digital watermark technologies can be applied to electronic advertisements (digital signage). For example, by detecting digital watermark information from a moving image obtained by capturing, with a camera, an advertisement motion picture with digital watermark information embedded therein shown on a street screen or a television screen, a user can obtain additional information such as detailed information of a commercial product introduced in the advertisement motion picture.
The content in which a digital watermark is embedded is sometimes degraded because of data compression processing, partial cutout of data, or filtering, for example. Therefore, the embedded digital watermark preferably has a resistance to degradation of data so as to be able to be detected by a digital watermark detecting device even if such data degradation occurs. Particularly in cases where a digital watermark is used for the purpose of protection for copyright, it is preferable that a digital watermark be able to withstand such an attack as one in which a malicious user analyzes a digital watermark and invalidates the digital watermark.
One of such attacks for invalidating a digital watermark is called a “collusion attack”. In an example of the collusion attack, a plurality of users separately obtain contents that are identical and in which different digital watermarks are embedded, and compare the contents to one another, so that the digital watermarks are analyzed. For example, data of portions corresponding to one another of the contents that are identical and in which different digital watermarks are embedded is averaged, which results in creating the content with the degraded digital watermark.
Digital watermarks having resistance to such a collusion attack are being studied.
For example, D. Boneh and J. Shaw, “Collusion-Secure Fingerprinting for Digital Data”, in Proc of CRYPTO'95, LNCS 963, 1995, pp. 452-465″ and G. Tardos, “Optimal Probabilistic Fingerprint Codes”, in STOC'03, ACM (2003), 2003, pp. 116-125″ has proposed a collusion-secure code with which, when c users make a collusion attack on data with a digital watermark embedded therein, there is a probability equal to or less than ε that digital watermark information for a user who is not a member of the collusion is detected. Unfortunately, the lengths of collusion-secure codes proposed in the above documents are very long and therefore are difficult to use. Japanese Laid-open Patent Publication No. 2007-178857 proposes a technique of reducing the lengths of collusion-secure codes. In this technique, a plurality of candidates of a threshold used for tracing an authorized user are generated by calculation, and a plurality of code lengths corresponding to the respective ones of the plurality of threshold candidates are obtained by calculation. Of the values of the plurality of code lengths, the smallest value is selected. Using the selected code length, codewords that differ from one user to another are generated.
Japanese Laid-open Patent Publication No. 2007-324720 proposes a technique of adding redundant bits to digital watermark information to generate an error correction codeword and embedding the error correction codeword in the content. It is also proposed that, for the error correction codeword, a plurality of pieces of scramble data is generated by using a plurality of different keys, and each scramble data is embedded as individual digital watermark information in the original content.